The term "chemotherapy" simply means the treatment of disease with chemical substances. The father of chemotherapy, Paul Ehrlich, imagined the perfect chemotherapeutic as a "magic bullet"; such a compound would kill an invading organism without harming the host. This target specificity is sought in all types of chemotherapeutics, including antimicrobial and anticancer agents.
Unquestionably, the greatest success with antimicrobials in terms of specificity has been with antibiotics. The antibiotic penicillin is widely known for its ability to block the synthesis of the cell wall for particular bacteria without interfering with the biochemistry of mammalian cells. What is not widely known is that penicillin is the exception rather than the rule; only a fraction of the thousands of identified antimicrobial drugs are non-toxic to humans.
Efforts to treat viral infection have been largely ineffective for precisely this reason. While a virus is essentially nothing more than nucleic acid surrounded by a lipid-protein envelope, a virus invades a host cell and uses the host cell's machinery to replicate itself. The latter characteristic makes it especially difficult to find drugs which block viral replication and yet leave intact the ability of the host cell to replicate.
Specificity has also been the major problem with anticancer agents. In the case of anticancer agents, the drug needs to distinguish between host cells that are cancerous and host cells that are not cancerous. The vast bulk of anticancer drugs are indiscriminate at this level. For this reason, only a few types of cancer are appropriate for chemotherapy. Surgery and radiation continue to be the favored types of cancer treatment.